Electrical insulating structure and method of making the same



May 21, 1957 P. DolGAN 2,792,871 ELECTRICAL TMSULATTNC STRUCTURE AMDMETHOD A 0E MAKING THE SAME Filed May 2T, 1955 'fm/627251 Paal Dogan, gyiwf M His frqg;

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ELECTRICAL INSULATING STRUCTURE AND METHOD OF MAKING THE SAME PaulDoigan, Pittsfield, Mass., assignor to General Electric Company, acorporation of New York Application May 27, 1955, Serial No. 511,601

3 Claims. (Cl. 1542.6)

The present invention relates to electrical insulating material, andmore particularly to resin-impregnated insulation structures forelectrical apparatus such as transformers, and particularly for dry-typetransformers.

The invention is especially concerned with resin impregnated asbestosinsulating sheet material which is wound in the form of cylinders foruse as dielectric barriers on transformer cores, e. g., for insulatingthe wound transformer coils from the core and from each other. Suchdielectric barriers especially when used in transformers and otherelectrical apparatus of high capacity must withstand high voltage andhigh temperature conditions without breaking down. Prior types ofinsulating cylinders made of inorganic base material and particularly ofasbestos have not proved satisfactory for many applications due to theirlow dielectric strength and poor physical and electrical properties onaging tests. Other types of known insulating sheet materials which haveimproved electrical properties such as higher dielectric strength haveproved unsatisfactory in `that they are diflicult to handle and windinto the desired form, due to their stiff and brittle quality, and moreover when wound into cylinders tend to lose the good electricalcharacteristics which they have in other insulation forms.

It is, therefore, an object of the present invention to provideelectrical insulation products which are suitable for high temperature,high voltage applications, and which overcome the disadvantages of knowntypes of such insulation.

It is another object of the invention to provide an improved insulatingbarrier of the above type which has superior electrical and mechanicalstrength properties even under prolonged high temperature and highvoltage conditions, and which can be readily wound into cylindrical formwith conventional winding devices.

It is still another object of the invention to provide a method ofmaking wound insulating barriers of the above type composed of asbestosbase material impregnated with a polyester resin material.

It has been found in accordance with `the present invention that woundinsulation products of superior electrical and physical characteristicsmay be made by impregnating asbestos base material containing organiccompounds (hereinafter also referred to as organic type asbestosmaterial) with a polyester resin composed -of a mixture of diallylphthalate and diethylene glycol maleate, the resin components beingpreferably present in 4approximately equal parts.

The invention will be better understood from the following descriptiontaken in conjunction withl the accompanying drawing, in which:

Fig. l schematically shows in partly sectional view a transformer corehaving arranged thereon cylindrical coil insulation barriers to whichthe present invention vis applicable; and

Fig. 2 is a fragmentary view of la lpartially 'nrolled insulatingcylinder of the type shown iii Fig. 1.

United States Patent O rice Referring now to the drawing, there is showna transformer core member 1 having an inner insulating cylinder 2arranged thereon, around which one set of coil windings 3 is wound.Surrounding the inner cylinder 2 and windings 3 is an outer insulatingcylinder 4 similar in structure to that of cylinder 2 and having woundthereon another set of windings 5. In practice, the sets of coilwindings are often placed on their respective cylinders before thecylinders are assembled on the transformer core, and then the coilwoundcylinders are slipped over the end of core leg 1 before the latter isjoined with the upper yoke 6. The illustrated insulating arrangement isadapted for use, for example, in a drytype ventilated distributiontransformer, although it is to be understood that the invention is notlimited to this type of transformer or to the arrangement illustrated.

The cylinders, one of which is shown in detail in Fig. 2. are normalyformed in known manner on conventional cylinder winding machines byWinding the resin-impregnated asbestos sheet, which may have a thicknessof about 0.010, into a multi-ply cylinder of tubular form of the desireddiameter and of a wall thickness of about one-quarter inch. A typicalprocess of fabricating such cylinders is set forth in detailhereinafter.

The mixture of diallyl phthalate and diethylene glycol maleate which isused as an impregnant in the present invention and which has been foundto provide insulating cylinders of outstanding properties, is of a classof resins which are 100% polymerizable. Upon curing, the present resincomposition solidifies without the evolution of water or water vapor asdo the so-called condensation curing plastics. The present copolymerdoes not evolve deleterious volatile solvents or thinners during theheat curing cycle, and the mass impregnating the organic type asbestossheet cures to the solid state without the formation yof bubbles,cracks, or other voids which, in an insulating barrier are extremelyundesirable in that they lower the dielectric strength of the insulationand raise the noise level. This property is of particular advantage inthe winding of cylinder types of insulating barriers, since where thepreviously employed types of resin dielectric impregnants upon curingwould leave voids or separations between adjacent turns of the woundimpregnated sheets, the present 100% polymerizable resin forms acontinuous compact void-free binder between, as well as within, the turnlaminations. Moreover, the present resin composition is characterized byhigh dielec tric strength and is extremely resistant to hightemperatures while maintaining its superior electrical properties.

The asbestos base material used in accordance with the invention ispreferably one which contains a substantial proportion of organicmaterial, which makes the asbestos more adapted than the solelyinorganic types of asbestos for the handling and winding processesinvolved in making the wound insulating barriers described. Aparticularly suitable type of such base material for the presentinvention is a sheet material formed of chrysotile asbestos fibers andhaving a composition of about purified asbestos and 15% organic content.Not only does such asbestos material have a high dielectric strength,but also has the requisite pliability for the vprocess of winding it`into multi-ply cylinders. lt is furthermore especially compatible withthe diallyl phthalatefdiethylene -glycol maleate resin impregnantdisclosed herein and in 0.5%. The organic content of this asbestosmaterial is about 1/3 chlorinated starch and 1% bleached sulfite pulp.This highly refined and purified asbestos has considerably higherdielectric strength than the usual commercial asbestos which has amagnetic particle content of 'about 6% and a greater concentration ofother impurities which result in less satisfactory dielectric propertiesin the final composite insulating cylinder. The advantage of a materialwith a lower magnetic particle content, aside from the more uniformdielectric strength afforded throughout the material, is theconsiderably reduced risk of burnouts in the cylindrical type ofinslulation where it is likely that conducting paths will be formedthrough the wall of the cylinder where magnetic particles becomesuperimposed in the winding operation.

The following table shows a comparison of the properties of the Quinorgotype of asbestos used in the present invention and commercial asbestosof the type heretofore commonly used:

Supplementary electrical tests made using each of the above differenttypes of asbestos sheet impregnated with the diallylphthalate-diethylene glycol maleate mixture under factory conditionsshowed that the average dielectric strength of the impregnatedcommercial type asbestos was about 120 Volts per mil, while that of theimpregnated Quinorgo type was about 168 volts per mil.

Tests conducted on cylinders made of resin impregnated purified asbestosmaterial in accordance with the invention show that cylinders havingabout 250 mils wall thickness could withstand up to 55 kilovolts andthat additional cure improves the dielectric strength of the resintreated asbestos material. In these tests the puriiied organic typeasbestos sheet was impregnated with the following composition, inpercent by weight:

Diallyl phthalate-diethylene glycol maleate The cylinders thus made werecured on the following cycle:

50 C.-2 hours 125 C.-16 hours 75 C.-2 hours 150 C.-- 2 hours 100 C.--2hours 200 C.- 2 hours The cured cylinders were then subjected to agingtests at 200 C., in which the following results were obtained:

As shown by the above data, the dielectric properties of theresin-impregnated asbestos cylinders not only were excellent initiallybut increased to considerably higher values after prolonged aging atelevated temperature.

It is to be noted that while solvents such as toluol and alcohol may beused, as in the above composition, for controlling the viscosity oftheresin to provide the most effective and uniform impregnation of theasbestos sheet, practically all of the thus added solvent is driven offbefore the final curing of the impregnated sheet, so that theundesirable effects of volatilization during curing, as discussed above,are avoided.

In making fiat laminated, resin-impregnated insulating structures forvarious electrical uses, it is relatively easy to provide effectivedielectric properties in the finished product even where ordinary resinsand insulating base materials are employed, due to the fact that thehigh molding and laminating pressures used in making such at productsproduce a compact, dense structure which resists arcing 'and voltagestresses, as well as a mechanically strong laminate. In the process offorming wound insulating cylinders, however, it is not feasible innormal winding operations to use such high pressures and as a result theprevious wound types of resin-impregnated insulating structures have nothad satisfactory dielectric properties, particularly 'at elevatedtemperatures. By virtue of the present invention, however, the cylinders`made of the materials disclosed may be formed on the conventionalwinding and tube rolling devices with the application of little or nopressure, and yet are characterized by superior properties in terms ofmechanical and thermal strength and excellent stability under highvoltage conditions.

The following is a typical procedure for making cylindrical insulatingbarriers in accordance with the invention.

Puried asbestos paper sheet of the Quinorgo type, .010 in thickness, isimpregnated with a polyester resin solution on a vertical treatingmachine. The impregnating solution is prepared by mixing the followingmaterials:

Resin mixture diallyl phthalate, 50% diethyl- The consistency of theabove solution may be altered by suitable variation of the type oramount of the solvents used to allow for the use of different treatingmachines. Further, the particular catalyst used is not critical, and anyof the known polymerization catalysts for polyester resins of the typeused can be employed.

The asbestos paper is passed into the treating solution in such manneras to allow for dotation prior to dipping, and the treated paper is thenconducted through a vertical infra-red drying tower. The speed oftreatment iu the solution and the drying conditions are governed by theresin content and condition of the treated material. The treated sheetmust be flexible and tack-free and should preferably have a resincontent of labout 30-40%. The thus treated paper is then ready forfabrication into a cylinder. For this purpose, the treated paper iswound on a rotating steel mandrel of the desired diameter, until acylinder of the desired wall thickness is obtained. For a Wall thicknessof M1, about 25 layers are required. While the treated paper is beingwound, a pool stock of the resin mixture is placed on the paper in thenip between the mandrel and the bed rolls, on which the mandrel restswhile rotating, and is wound into the cylinder turns.

The mandrel containing the wound cylinder is then removed 'from the tuberolling machine, and placed in an oven where it is given a controlledprogram cure, such as described hereinbefore. After curing, the cylinderis stripped from the mandrel and cut and sanded to the proper dimensionsfor use.

While the `present invention has been described with reference `toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations as comeWithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Electrical insulating structure having high dielectric and mechanicalstrength and high temperature stability comprising a wound multi-plycylindrical product adapted for use as an insulating barrier forelectrical apparatus, said cylindrical product being composed ofinsulating sheet material consisting in per cent by weight of about80-85% asbestos bers, about 15-20% organic material, and a maximum ironcontent of about 0.5%, said insulating sheet material being impregnatedwith a copolymer of about equal parts of diallyl phthalate anddiethylene glycol maleate.

2. Electrical insulating structure having high dielectric and mechanicalstrength and high temperature stability comprising a wound multi-plycylindrical product adapted for use as an insulating barrier forelectrical apparatus, said cylindrical product being composed ofinsulating sheet material consisting in per cent by weight of about80-85% asbestos bers and about 15-20% organic material, said insulatingsheet material being impregnated with a copolymer of about equal partsof diallyl phthalate and diethylene glycol maleate.

3. The method of making an electric insulating structure which comprisesimpregnating lasbestos sheet material composed in per cent weight ofabout 80-85% asbestos bers and about 15-20% organic material with aresin mixture composed of about equal parts of diallyl phthalate anddiethylene glycol maleate, winding the thus treated asbestos sheetmaterial into a multi-ply tubular structure, and curing the resinimpregnant to a hardened state.

References Cited in the tile of this patent UNITED STATES PATENTS2,443,739 Kropa June 22, 1948 2,443,741 Kropa June 22, 1948 2,530,983Minter Nov. 21, 1950 2,542,819 Kropa Feb. 20, 1951 2,581,862 Johnson etal. Ian. 8, 1952

1. ELECTRICAL INSULATING STRUCTURE HAVING HIGH DIELECTRIC AND MECHANICALSTRENGTH AND HIGH TEMPERATURE STABILITY COMPRISING A WOUND MULTI-PLYCYLINDRICAL PRODUCT ADAPTED FOR USE AS AN INSULTING BARRIER FORELECTRICAL APPARATUS SAID CYLINDRICAL PRODUCT BEING COMPOSED OFINSULATING SHEET MATERIAL CONSISTING IN PER CENT BY WEIGHT OF ABOUT80-85% ASBESTOS FIBERS, ABOUT 15-20% ORGANIC MATERIAL AND A MAXIMUM IRONCONTENT OF ABOUT 0.5%, SAID INSULATING SHEET MATERIAL BEING IMPREGNATTEDWITH A COPOLYMER OF ABOUT EQUAL PARTS OF DIALLYL PHTHALATE ANDDIETHYLENE GLYCOL MALEATE.